Method and apparatus for adhesion testing of thin film materials

ABSTRACT

An apparatus for the automatic testing the adhesion characteristics of thin film and coating to silicon wafers, and the processing of the data obtained thereby to present to the user a rapid and accurate forecast of the thin film&#39;s behavior in a selected processing environment. The apparatus of the invention heats and cools samples while automatically monitoring for debonding. Information collected from the optical and thermal devices are processed by computer for analysis and recorded for cataloging. Information is collected and processed over time while samples are subjected to selected temperature environments to provide a data base of adhesion characteristics of thin films and coatings.

BACKGROUND OF THE INVENTION

The present invention relates to material testing. More Particularly,the present invention relates to the testing of adhesion properties ofthin film materials.

The microelectronics industry is ever increasing the density ofcomponents to meet cost and performance demands of the consumer.Reliability and service life are of concern to both the industry andcustomers. One technology trying to meet those demands is thin filmtechnology. The industry is researching the application of various newmaterials to meet the low dielectric constant requirement for improvedback-end of line interconnect performance.

Successful application of the new materials requires maintainingmechanical integrity, including fracture resistance, throughout themultilayering processes. The adhesion of grown and deposited films mustbe excellent to ensure good reliability and service life of theresulting products. In most cases, adhesion is strongly affected by thecleanliness of the substrate. Contamination of interfaces results inreduced adhesion, as does an adsorbed gas layer. Substrate surfaceroughness can promote adhesion, but may also result in coating defects.

Adhesion of thin films and coatings to various materials is ofimportance when they are subjected to various conditions or processes.Because electronic devices are fabricated from a variety of materialsusing silicon substrates and various applied films, stresses developbetween layers as a result of expansion mismatches between thelaminations. Poor adhesion represents a potential reliability problem.If films lift from the substrate, device failure can result.

The adhesion of thin films is particularly important in themicroelectonics industry where devices are subjected harsh conditions.With the industry drive toward smaller, higher speed devices, materialsfor back-end of line processes are needed to reduceresistance-capacitance time delay in next generation integratedcircuits. Hence, process integration now involves new thin filmmaterials such as low k, copper and other novel materials.

Several materials are prone to delaminate or exhibit detrimentalphysical and material changes upon heating and cooling. Problems arecompounded by the requirements of multiple layers of coatings or thinfilms. Consequently manufacturers are requiring greater adhesionspecifications. Failing to meet these greater specifications canpreclude the manufacturer's product from entering the marketplace. Thus,data on the mechanical reliability of these new materials is critical.However, since such data is not available, sufficient test data needs tobe developed to aid in the prediction of a product's use limit orlifetime and reliability.

Adhesive failure can be predicted when applied energy exceeds a criticalfracture property of a union. The demand in designing and measuringadhesion is to establish the characteristics of both the appliedenergies and the critical performance properties. Performance propertiesvary with a myriad of processing and environmental conditions, hence,any test developed to measure these properties must be capable ofsimulating the same processing conditions.

Of great concern is adhesive failure due to large thermal stressesdeveloping during processing. To be able to predict the behavior of adesign, failure data must be available to compare to known stressfields.

Early attempts to measure adhesion included the use of the tape test anda method of abrasion. The tape method consisted of pressing a piece ofadhesive tape to the film. The tape is then pulled off the film eitherleaving the film intact, removed in whole or in part, or remaining onthe substrate. This method is qualitative only, and if the film remainson the substrate, it provides no quantitative data as to the magnitudeof the adhesion forces. Failure of the tape test implies that the filmis unsuitable for device fabrication.

The abrasion or scratch test method results depend on the film hardnessas well as on its adhesion. All of these conventional tests for adhesionare qualitative at best and do not accurately model the fracturemechanism.

The modified Edge Liftoff Test has been developed and applied to testingmultilevel coatings on a rigid substrate. This test is applicable totesting the adhesiveness of multilayers of microelectronic structures inthat it allows testing of samples constructed with standard back-end ofline processes.

A thick coating of epoxy is applied to a multilayer device. Failure ofthe adhesive forces is caused by the stored strain energy in the thickepoxy layer exceeding the critical adhesion energy of the weakestcomponent interface as the test sample is cooled. Advantages of thistype of testing are simplicity and resultant true fracture energies ofthe system. Reliability of the device can be quickly assessed bycomparing the measured fracture energies to the calculated appliedfracture energies from finite element analyses. However, this systemgenerally requires a human observer to continually peer through glassplates to monitor delamination of the films.

In view of the above, what is needed is a modified edge lift test systemcapable of providing consistent data of the adhesion characteristics offilms and coatings. The system should be capable of sealed automatictemperature processing, including computer controlled delaminationdetection and temperature control.

SUMMARY OF THE INVENTION

To address current industry needs, the invention offers a modified edgelift test system which includes a sealed automatic temperatureprocessing type chamber that operates in a computer controlled heatingand cooling mode, and a computer processor with delamination detectingsoftware. The invention's ability to provide consistent data of theadhesion characteristics of old and new films and coatings can providevaluable processing data to the industry and speed up the manufacturingprocess. This invention is particularly applicable to industries such assemiconductor manufacturing where the search for new thin film materialsand coatings is ongoing.

The invention provides for an improved system for quantitative andqualitative testing of the adhesion characteristics of thin filmmaterials and coatings during thermal processing. In one embodiment, anapparatus of the present invention includes an atmosphere sealablechamber set within a metal housing. An optical window made of atransparent temperature stable material such as Plexiglas, glass orquartz mounted in a wall of the chamber to view the samples beingtested. A sample tray holder, capable of holding multiple samples,inserted though a side wall of the chamber and lockable into position. Acamera is mounted on top of the optical window to replace a humanobserver in monitoring the testing process. Lighting is mounted adjacentto the improved optical window to provide a sufficient amount of lightto illuminate the samples being tested for observation and recordation.A computer processor, with image processing software, is connected tothe testing apparatus and the camera for collection and analysis oftemperature and visual data gathered during the testing process.

In another embodiment, a sample tray is inserted into the chamberthrough a slot in the wall of the chamber and sealed with clamps. In yeta further embodiment, a single viewing window made of a solidtemperature stable material is used to address the problems of shadows,ghost images, double reflection, and frost.

In one aspect of the invention, two lamps, one mounted on two differentsides of the viewing window are utilized to provide a constant andsufficient amount of light for optical viewing and recording by a cameramounted on top of the viewing window. The lamps provide a ghost free,shadow free, viewing and recording of sample images.

Advantageously, the present invention provides more efficient thermaltransfer by using an improved heat exchanger where the heating/coolingsurface has raised posts. Moreover, the present invention uses anautomated gas and liquid nitrogen mixer assembly. Through the use ofgages or reducers and valve configuration, the computer controls theopening or closing of valves to set the flow of gas and liquid nitrogeninto proper ratios prior to introduction into the chamber for either aspecified temperature range or time period.

Another advantage of the present invention is the use of a digitalcamera to record an image change in the place of a human operator andcomputer imaging software which captures and records debonding ofsamples and the temperatures at which debonding occurs and thencalculates the debonding energies. In addition, the present inventionprovides complete automation and control of the entire process bycomputer, from heating through cooling and calculation of debondingenergies, with graphing of data in a visual chart for the user.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention together with further advantages thereof, may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a pictorial view of an apparatus for testing the adhesioncharacteristics of thin film materials and coatings during thermalprocessing in accordance with one embodiment of the present inventionwhich includes the testing chamber, a temperature control unit and acomputer processor;

FIG. 2 is a pictorial view of the sample tray in accordance with anembodiment of the present invention;

FIG. 3 is a pictorial view of the optical window in accordance withanother embodiment of the present invention;

FIG. 4 is a pictorial view of the mounted lighting in accordance with anembodiment of the present invention;

FIG. 5 is a pictorial view of the heat exchanger in accordance with anembodiment of the present invention;

FIG. 6 is a pictorial view of the gas and liquid nitrogen mixer assemblyin accordance with an embodiment of the present invention;

FIG. 7 is a pictorial view of the gas and liquid nitrogen mixer and agas flow gauge in accordance with an embodiment of the presentinvention;

FIG. 8 is a flow chart showing a method for measuring the adhesioncharacteristics of thin film materials and coatings during thermalprocessing according to one embodiment of the invention;

FIG. 9 is a flowchart showing a method for data collection in accordancewith an embodiment of the present invention;

FIG. 10 is a flow chart showing a method for processing and analyzingdata from tests performed on samples in accordance with an embodiment ofthe present invention; and

FIG. 11 is a flow chart showing a method for temperature control duringthe testing of samples in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

An invention is disclosed for an improved system for quantitative andqualitative testing of adhesion characteristics of thin film materialsand coatings during thermal processing. In the following description,numerous specific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art, that the present invention may be practicedwithout some or all of these specific details. In other instances, wellknown process steps have not been described in detail in order not tounnecessarily obscure the present invention.

FIGS. 1-7 illustrate an apparatus of the preferred embodiment of animproved system for quantitative and qualitative testing of the adhesioncharacteristics of thin film materials and coatings during thermalprocessing.

The invention includes a housing; an atmosphere sealable chamber 6capable of achieving multiple temperatures; an improved optical window 3made of a transparent temperature stable material such as Plexiglas,glass or quartz mounted in a wall of the chamber, a sample tray holder 4inserted though a wall of the chamber and lockable into position, a heatexchanger/mixer 12, a thermocouple 1, a camera 9 and lighting 2 mountedadjacent to the improved optical window to monitor the samples duringtesting, mixing assembly 8, control valves 17 and gages 16 for mixinggas and liquid coolant, a temperature control unit 5 and a computerprocessor 7, with image processing software, connected to the testingmachine and camera for collection and analysis of temperature and visualdata.

Additionally the invention can be fitted with an acoustic and/or a laserdetector sensing devices for gathering temperature and delaminationdata.

In practice, prior to testing, a sample silicon wafer is first measuredfor thickness with a micrometer. Next epoxy is placed on the wafer toact as a backing layer over the thin film so that the point ofdelamination can be more easily observed during testing. To ensure goodadhesion between the epoxy and the test layer, an epoxy, chosen for itsgood wetting, adhesion, and to provide sufficient energy to causedelamination, is applied with an epoxy applicator to the wafer, to ameasurable thickness of approximately 100 microns. The actual thicknessof the thin film is so proportionately thinner than the thickness of theepoxy as to be negligible in terms of the film's thickness measurementcompared to the epoxy thickness.

The wafer with an epoxy coating is then cured in an oven for about anhour.

The cured epoxy coated silicon wafer sample to be tested is firstdivided into rectangular portions to obtain edges that are 90 degrees tothe interface. Each square sample is measured with a micrometer at eachof its four corners for thickness and recorded. The rectangular portionsare placed on the sample tray 4 in the sample spots 13.

The invention has a sample tray which is inserted into the chamberthrough a slot in the wall of the chamber and sealed with clamps 63. Thesample tray has convection accelerator holes 14 placed around the samplespots 13 to more efficiently effect cooling. The sample tray is thenplaced in the testing chamber 6.

A temperature control unit 5 is used to control heating and cooling bycontrolling the coolant circulating through the heat exchanger/mixer 12.The samples may be heated to an elevated temperature of about 100degrees Centigrade by the heat exchanger/mixer 12 in the chamber. Anadvantage of the invention is the improvement in the heatexchanger/mixer 12 where the heating/cooling surface has raised posts 10for more efficient thermal transfer.

Chamber temperature and thermal cycling is monitored by a thermocouple,located near or contacting a sample or sample tray, and is controlled byan electrical temperature control unit 5. Samples are electricallywarmed by the heat exchanger upper body plate 18 to a selectedtemperature. Samples are cooled by the heat exchanger/mixer 12 by theintroduction of a cooling gas and liquid nitrogen mixture, from themixer assembly 8, mounted below the sample tray, which is fed by gaslines from a cylinder of liquid nitrogen, until fracture is observed.

The heat exchanger/mixer 12 is mounted below the sample tray 4 toprovide a heating and cooling source for varying the temperature of thesamples during thermal cycling. The heat exchanger/mixer is electricallywired for heating by electrical resistor 15. Heating and cooling isfacilitated through the heat exchanger/mixer 12 by the raised posts 10inserted in the heat exchanger upper body plate 18.

The heat exchanger/mixer 12 consists of a heating element 15 for heatingsamples to an elevated temperature, and a mixing chamber where gas andliquid nitrogen are mixed by the gas/fluid mixers to cool the sampleuntil fracture is observed. The mixture of gaseous and liquid nitrogenis controlled by valves 17 which control the amount of coolant into thegas/liquid nitrogen mixer 11 which permits efficient controlled coolingof the testing chamber.

An advantage of the invention is the automated gas and liquid nitrogenmixer assembly 8. Through the use of gages 16 or reducers and valve 17configuration, the computer controls the opening or closing of valves toset the flow of gas and liquid nitrogen into proper ratios prior tointroduction into the chamber for either a specified temperature rangeor time period.

The amount of cooling mixture of gas and liquid nitrogen comes from themixer assembly 8, mounted below the sample tray, which is fed by gaslines from a cylinder of liquid nitrogen, and is controlled by valves 17and gages 16 before entry into the heat exchanger/mixer 12. The mixerassembly 8 combines liquid and gas nitrogen into the proper ratiosoutside of the chamber and is passed into the heat exchanger/mixerinside of the chamber through the predistributors 11 inserted in theheat exchanger/mixer lower body plate 19.

Improved optical viewing is accomplished through a window 3 on top ofthe chamber made of a one-piece solid transparent temperature stablematerial such as quartz, glass, Plexiglas or suitable transparentmaterial. The invention has resolved the problems of shadows, ghostimages, double reflection and frost problems caused by multiple glasspaned windows, by utilizing a single viewing window made of a solidtemperature stable material. The solid window removed the additionalburden of passing warm nitrogen gas over the multiple glass paned windowto remove frost. The solid window removed the double reflection andghost images caused by the use of multiple plates of glass. A clear,transparent optical window 3 is mounted in the chamber wall above thesample tray for optical scanning by camera 9 or human eye.

The invention has lighting consisting of two lamps 2, one mounted on twodifferent sides of the viewing window to provide a constant andsufficient amount of light for optical viewing and recording by a camera9 mounted on top of the viewing window. The camera removes therequirement of a human observer continually peering through the windowto monitor debonding.

The lamps provide a ghost free, shadow free, viewing and recording ofsample images. The lamps 2 for illuminating the sample tray are mountednext to the camera. A thermal couple 1 is mounted within the chamber tomeasure temperature. The camera takes snapshots of the sample tray ateach one degree drop in temperature. As the samples cool, the camerarecords the moment of fracture of the film from the substrate and thecomputer processor captures the event with its image processing softwareand records the temperature at the moment of debonding.

An advantage of the invention is the use of a digital camera to recordchanges in place of a human operator and computer imaging software whichcaptures and records debonding of samples and the temperatures at whichdebonding occurs and then calculates the debonding energies. Informationcollected from the temperature control unit 5 and camera 9 is collectedand analyzed by the computer processor 7 to provide an analysis of theadhesion characteristics of each sample being tested during the thermalcycling.

A computer 7 and camera system with image imaging software is used tocontrol and record the entire heating/cooling process either over aspecified time period or over a specified temperature range, at onedegree temperature increments until the moment of fracture ordelamination of each sample, as well as recording the temperature at themoment of fracture or delamination 64.

FIG. 8 is a flow chart showing a method 900 for measuring the adhesioncharacteristics of thin film materials and coatings during thermalprocessing according to one embodiment of the invention. Samplepreparation 901 begins with cleaning and measuring of samples forthickness, the application of epoxy, curing the epoxy, portioning thesamples and measuring the portioned epoxy coated samples. Samplethickness data is stored. The samples are loaded 902. The user decidesthe setup parameters 903 and starts the experiment 904.

Data is collected 905 and a decision is made to either save the data tostorage 906 or process and analyze 908 the data. At the completion ofdata analysis a report 909 is generated with the test results.

FIG. 9 is a flowchart showing the steps of data collection asillustrated in FIG. 8. Data collections begins with collecting imagesfrom the image capturing device 910. Next temperature and time data fromsensors is collected 911 and saved to storage 912. A decision is madewhether to continue 913 the experiment to conclusion with the saved dataor repeat the data collection process. Data is captured at each onedegree increment of temperature drop.

FIG. 10 is a flow chart showing a method for processing and analyzingthe data from tests performed on samples as illustrated in FIG. 8. Indata processing, a first image 914 of the samples is collected at atemperature T1 and a second image 915 is collected at T2. Both imagesare compared using comparison filters 916 resulting in a third image917. Using morphological filters 918 on the third image results in afourth image 919.

Data analysis begins with feature extraction filters 920 applied to thefourth image resulting in a fifth image 921. Test results are thencalculated and a decision is made whether to process more images 923 orby repeating the data processing process again, or finish with a report909.

FIG. 11 is a flow chart showing the steps of temperature control toillustrate controlling the cooling rate during testing of samples.Temperature control begins with sensing a temperature T at time t 1000after at least 3 temperature increment drop. The temperature T iscompared to T-3 which is 3 degree increments before the currenttemperature T. By comparing the two sets of time and temperature, acooling rate is calculated 1001. If the calculated cooling rate isgreater than 6 degrees per minute 1002, cooling is slowed by closing acontrol valve 1003, and time and temperature are again compared 1000,1001. If the cooling rate is less than 1 degree per minute 1004, acontrol valve is opened 1005, and time and temperature are againcompared 1000, 1001. If the cooling rate is between 6 degrees per minuteand 1 degree per minute, cooling continues at that rate, and time andtemperature are again compared 1000, 1001.

An advantage of the invention is computer imaging software which is ableto simultaneously capture the images of the samples and record thedebonding of samples and the temperatures at which debonding occurs andthen automatically calculates the debonding energies. The camera 7 takessnapshots of the samples at each one degree of temperature change. Asthe samples cool, the camera records the moment of fracture of the filmfrom the substrate and the computer processor records the photo and thetemperature at the moment of fracture.

Another advantage of the invention is the complete automation andcontrol of the entire process by computer processor from heating throughcooling, image capturing and calculation of debonding energies, thenproviding the user with graphing of the resultant data in a visual chartfor the user.

Calculation of the amount of energy necessary to separate the film layerfrom the substrate is performed by the computer processor. Themeasurements of thickness of each sample are inputted into the computer.Using the temperature at which the delaminating occurs, calculation ofthe fracture energy needed for the thin film or coating to delaminate isaccomplished by extracting the applied stress value from the graph ofthe function of stress over temperature of the epoxy 65.

Characterization of the epoxy is established by stress testing an epoxycoated silicon wafer and graphing the data as a function of stress.Applied stress intensity (Kapp) is a function of stress ( ) overtemperature (t) and the square root of the thickness (h) of the epoxylayer divided by two, or: Kapp=(t) (h/2).

The recorded temperature at which delaminating occurs in the sample isused by the computer processor to calculate the critical fracturetoughness of the thin film or coating.

If a peeling site is selected, the image processing software willcalculate and show the amount of energy needed to separate the filmlayer from the substrate. An advantage of the invention is the completeautomation and control of the entire process by computer from heatingthrough cooling and calculation of debonding energies with graphing ofdata in a visual chart for the user.

The invention's achievement is that the entire process is automated andcontrolled by a computer processor 7 which is connected to thetemperature control unit 5, the camera 9 , the heat exchanger/mixer 12,and the mixer assembly 8.

While the present invention may have been described in terms of severalpreferred embodiments, there are many alterations, permutations, andequivalents which may fall within the scope of this invention. It shouldbe noted that there are many alternative ways of implementing themethods and apparatuses of the present invention. It is thereforeintended that the following appended claims be interpreted as includingall such alterations, permutations, and equivalents as fall within thetrue spirit and scope of the present invention.

What is claimed is:
 1. An apparatus for analyzing adhesioncharacteristics of thin films to materials comprising a housing; achamber disposed within said housing, said chamber being capable ofachieving multiple temperatures; at least one sample tray disposedwithin said chamber for holding material samples; a first and secondsensor disposed within said housing for providing a first set of sampleimages and second set of corresponding sample temperatures; and a datacorrelator coupled to said first and second sensors for correlating afirst sample image at a temperature T1 and a second sample image at atemperature T2, wherein T2<T1.
 2. An apparatus as recited in claim 1,further comprising at least one light mounted on said housing forilluminating material samples.
 3. An apparatus according to claim 2wherein said light source is an incandescent lamp.
 4. An apparatusaccording to claim 2 wherein said light source is a fluorescent lamp. 5.An apparatus as recited in claim 1, further comprising a temperaturecontrol unit coupled to said housing for controlling the temperature ofsaid chamber.
 6. An apparatus as recited in claim 5, further comprisinga heat exchanger/mixer disposed within said chamber capable of achievingmultiple temperatures for varying the temperature of said chamber.
 7. Anapparatus as recited in claim 6, further comprising mixing controlvalves coupled to said housing for mixing gas and liquid coolant to varythe temperature of said chamber.
 8. An apparatus as recited in claim 1wherein said first sensor is a thermocouple.
 9. An apparatus as recitedin claim 1 wherein said second sensor is an image capturing devicemounted on said housing for recording images of said samples.
 10. Anapparatus according to claim 9 wherein said image capturing device is acamera.
 11. An apparatus according to claim 9 wherein said imagecapturing device is a motion camera.
 12. An apparatus as recited inclaim 9 wherein said image capturing device is a digital camera.
 13. Anapparatus as recited in claim 9 wherein the data correlator is coupledto said image capturing device, a thermocouple, a heat exchanger/mixer,and mixing control valves.
 14. An apparatus as recited in claim 13wherein said mixing control valves are further coupled to said housingfor mixing gas and nitrogen to vary the temperature of said chamber. 15.An apparatus as recited in claim 1, wherein said data correlatorincludes image analyzing software for processing and correlating imagesof said samples.
 16. An apparatus as recited in claim 1 furthercomprising convection accelerator holes placed into the body of saidsample tray.
 17. An apparatus as recited in claim 1 further comprisingpredistributors inserted into said heat exchanger/mixer.
 18. Anapparatus according to claim 1 further comprising at least one opticalviewing window mounted in said chamber above said sample tray forviewing said material samples.
 19. An apparatus as recited in claim 18wherein said optical viewing window is made of a temperature stablematerial.
 20. An apparatus as recited in claim 18 wherein said opticalviewing window is made of Plexiglas.
 21. An apparatus as recited inclaim 18 wherein said optical viewing window is made of glass.
 22. Anapparatus as recited in claim 18 wherein said optical viewing window ismade of quartz.